Method of making composite structure with single domain magnetic element

ABSTRACT

A surface smooth enough to allow the formation thereon of a single domain magnetic element is obtained by covering a polished layer with a thin layer of the same material, to smooth sharp edges and corners. The resulting structure is useful in thin film magnetic heads.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 703,539, filed May 21, 1991.

BACKGROUND OF THE INVENTION

This invention relates to thin film magnetic heads, and moreparticularly relates to a composite structure including a substantiallysingle domain magnetic element, a method of producing such a structureand thin film magnetic heads incorporating such a structure.

Thin film magnetic heads are being developed which offer the advantagesof miniaturization and integration on a single substrate. In the case ofmagnetic tape, information is written onto, and read from, spaced,parallel tracks on the tape. To increase information density, the widthof the tracks, as well as the spacing between the tracks, can bereduced. For example, in the newly proposed format for digital (audio)compact cassettes (dcc), there are a total of 18 separate, paralleltracks on a tape having the same width as the conventional compactcassette.

In order to achieve magnetic heads having correspondingly smalldimensions, such heads are now being developed using thin filmprocessing techniques of the type used to manufacture integratedcircuits in silicon substrates.

In order to achieve track alignment between the read and write magneticheads, it has been proposed that the read and write heads be integrallycombined in a unitary structure. See, for example, parent U.S. patentapplication Ser. No. 703,539 (PHA 21,669), filed May 21, 1991, andassigned to the present Assignee, in which a combined read/writemagnetic head is disclosed in which the read head includes amagnetoresistive element (MRE) overlying a substrate of substantiallymagnetically impermeable material, a broken (discontinuous) flux guideoverlying the MRE and a continuous flux guide overlying the broken fluxguide. The write head of the combined read/write magnetic head overliesthe read head and shares the continuous flux guide with the read head,the continuous flux guide serving as a bottom pole of the write head.Significantly, the dimensions of the various elements of the combinedread/write magnetic head are chosen so that relatively little magneticflux produced by the write head during writing, or entering the writehead during reading, is communicated via the shared flux guide to theMRE.

The successful operation of such a device depends on the MRE of the readhead being a substantially single domain element. The ability to depositsuch an element requires a smooth, preferably stress-free substratesurface. Otherwise, surface irregularities can form pinning sites forthe formation of domain wall boundaries.

In U.S. Pat. No. 4,608,293, a magnetic layer is formed on an insulatingoxide layer after the surface of the oxide layer has been carefullypolished under pressure using a rotating polisher in a suspension inpure water of MgO, SiO₂, or Al₂ O₃ powder or a mixture thereof having aparticle diameter not exceeding 0.1 micron.

However, such a polishing techinque leaves surface scratches which canact as pinning sites for the formation of domain wall boundaries.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a method forproducing a thin film magnetic head including a substantially singledomain magnetic element, which method does not depend upon a carefullycontrolled polishing of the surface upon which the element is to beformed.

It is another object of the invention to provide a method of producing acomposite layer of an insulating material having sufficient surfacesmoothness to allow the deposition of a substantially single domainmagnetic element.

In accordance with the invention, a method of producing a compositestructure including a single domain magnetic element comprises the stepsof: depositing a relatively thick layer of an insulating material onto asubstrate; followed by depositing a relatively thin layer of the samematerial onto the relatively thick layer. The ratio of layer thicknessesmay, for example, be in the range of about 8 to 40. The purpose of therelatively thin layer is to conform to and smooth out surfaceirregularities in the surface of the relatively thick layer, and therebyproduce a surface of sufficient smoothness to support the formation of asubstantially single domain element such as an MRE.

For example, where the surface of the relatively thick layer hasindentations with sharp edges and/or corners, such as might be formedduring conventional mechanical lapping or polishing techniques, therelatively thin layer conforms to and smooths out these sharp edges andcorners, which could otherwise form pinning sites for domain wallsduring the subsequent deposition of the magnetic element, leading to amulti-domain structure. Preferably, the thickness of the relatively thinlayer is in the range of from about 2 to 10 times the average depth ofthe surface indentations.

Selection of the same material for both the thick and the thin layerinsures to a large extent chemical and physical compatibility, andconsequently promotes an adherent, yet stress-free bond between thelayers. Preferably, the layers are both deposited by the same technique,thereby further promoting such compatibility.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described with reference to the accompanyingdrawing, in which:

FIG. 1 is a diagrammatic cross sectional view of a portion of acomposite structure, produced in accordance with the method of theinvention, including a substrate and a composite insulating layersupporting a substantially single domain magnetic element; and

FIG. 2 is a diagrammatic cross sectional view of one embodiment of acombined read/write thin film magnetic head incorporating the compositelayer structure of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a portion 10 of a composite structure of thepresent invention includes a substrate 12 of substantially magneticallyimpermeable material, such as the material sold under the tradenameAlsimag by 3M of Minneapolis. (For purposes of the present invention, amaterial is substantially magnetically impermeable if its permeabilityis less than about 2.) As is known, the composition of Alsimag includesTiC as well as Al₂ O₃, and has the nominal composition in weightpercent: 70 Al₂ O₃, 30 TiC. The actual composition generally fallswithin the range 63-70 Al₂ O₃, 27-30 TiC, up to 5 ZrO₂, with theremainder being made up of minor impurities, such as Mg, Ca, Fe, Co, Ni,W, Mo, and Y₂ O₃.

When purchased, the surface of an Alsimag substrate is usuallynonplanar, i.e., the surface typically includes 5 micrometer (μm)-deepconcavities 13, called "pullouts". In order to provide a substantiallyplanar surface on which to deposit the single domain magnetic element18, and to provide electrical insulation between element 18 and theAlsimag substrate 12, a relatively thick layer 14 of electricallyinsulating and substantially magnetically impermeable material, such asa layer of Al₂ O₃, is deposited, e.g., sputter deposited, onto theAlsimag substrate. (A material is electrically insulating, for purposesof the present invention, if its resistivity is greater than about 10⁶μΩ-cm.) The initial thickness of the layer 14 is 14-20 μms.

After being deposited, the surface of the layer 14 is polished toachieve a relatively smooth surface on which to deposit the element 18.The resulting thickness of the layer 14 is 8-15 μms. In a typicalpolishing technique, a two stage planetary process is used, in which thesurface is given a rough lap and a finish lap. About 80 percent of thematerial removed by lapping from the layer is achieved by rough lapping,and the final 20 percent by finish lapping. By way of example, thesubstrate with a 20 μm thick layer of Al₂ O₃ is mounted on a fixture,and the fixture is rotated about its axis while bringing the surface ofthe layer into contact with a 40 inch diameter lapping wheel, which isalso rotating on its axis. In the first stage, the polishing compound is6 μm diamond particles, and the wheel is a copper alloy. In this stage,the thickness of the layer is reduced from 20 μto about 14 μm. In thesecond stage, the polishing compound is 1 μm Al₂ O₃ and the wheel is afabric pad. In this stage, the thickness of the layer is reduced to 12microns. Unfortunately, such conventional polishing techniques usuallyleave 0.5 μm--deep, sharp-cornered scratches (indentation) 15 in thesurface of the layer 14, which are undesirable because the sharp cornersserve as pinning sites for magnetic domain walls, resulting in a multidomain element 18. To smooth out the sharp corners, a relatively thinlayer 16, e.g., from 0.75 to 1.5 μm in thickness, of Al₂ O₃, isdeposited, preferably sputter deposited, onto the layer 14. Layers 14and 16 together comprise a composite insulating layer of the invention.

A substantially single domain MRE 18 of magnetically permeable material,such as permalloy, is deposited directly on the layer 16. (Amagnetically permeable material, for purposes of the present invention,is one having a permeability equal to or greater than about 100. Inaddition, a substantially single domain MRE, for purposes of the presentinvention, is one which has no domains in the active area. By contrast,a destabilized or multi-domain MRE, for purposes of the presentinvention, is one which has domains in the active area.) If the MRE 18is of permalloy, then the composition of the permalloy is, for example,18-22 percent Fe and 82-78 percent Ni. The MRE 18 is readily deposited,in the form of a substantially single-domain element, onto the layer 16using conventional techniques, such as the magnetron sputter depositiontechnique. Layer 18 completes the composite structure 10 in accordancewith the invention.

With reference to FIG. 2, a preferred embodiment of a thin film magnetichead incorporating the composite structure of the present invention, acombined read/write head 210 includes a substrate 220, insulating layers230 and 240, and MRE 250, corresponding to layers 12, 14, 16, and 18,respectively, of FIG. 1. In this embodiment, the thickness of the MRE250 is between about 0.025 and 0.08 μms, preferably 0.03 to 0.035 μms.

The MRE 250 is preferably provided with a barber pole configuration ofconductive strips 260, one of which is shown in FIG. 2. The conductivestrips include, for example, successive layers of Mo, Au, and Mo, havingthicknesses of, respectively, 0.03 μm, 0.23 μm, and 0.09 μm. The widthof each conductive strip is in the range of about 2-6 μms, the spacingbetween successive strips is about 5-15 μms, and the angle between eachconductive strip and a longitudinal direction of the MRE, along whichthe MRE is magnetized, is 40°-50°. The barber pole configuration isachieved by first depositing continuous layers of Mo, Au, and Mo, usingconventional deposition techniques, e.g., conventional sputterdeposition techniques, and then patterning the layers into a barber polepattern using conventional etching techniques.

The barber pole produces a longitudinal aligning field coinciding withthe easy axis of magnetization of the MRE, thus insuring stability ofthe MRE during normal operation of the combined read/write head.

Electrical insulation for the conductive strips 260 is provided bydepositing a layer 270 of electrically insulating and substantiallymagnetically impermeable material, e.g., a layer of Al₂ O₃, onto theconductive strips using conventional deposition techniques. Thethickness of the layer 270 is about 0.3-0.6, preferably 0.35 μms.

The read head also includes a broken (discontinuous) flux guide,including flux guide sections 280a and 280b, overlying the electricallyinsulating layer 270. The flux guide sections are of magneticallypermeable material such as permalloy, the composition being, forexample, 19.2 percent Fe and 80.8 percent Ni. The flux guide sections280a and 280b are formed, for example, by initially depositing acontinuous flux guide layer and then etching away a central strip ofdeposited material, or by using conventional selective depositiontechniques. The thickness of each flux guide section is about 0.2-3.0μms, preferably about 0.4 μm.

As shown in FIG. 2, each of the flux guide sections 280a and 280bpartially overlaps the MRE 250 in order to couple flux into the MRE.

The read head also includes a read gap defined by a layer 290 ofsubstantially magnetically impermeable material, e.g., a layer of Al₂O₃, deposited onto the broken flux guide using conventional techniques.The thickness of the layer 290 is about 0.2-0.9 μms, preferably about0.38-0.4 μms.

Preferably, the read head includes a test/biasing electrical conductor300 overlying the read gap layer 290. (An electrical conductor, forpurposes of the present invention, is a structure which includesmaterial having an electrical resistivity equal to or less than about100 μΩ-cm.) This conductor 300 is useful, for example, in generatingmagnetic fields for testing the MRE 250, and for generating a magneticfield for biasing the MRE 250, so as to linearize the signal output fromthe MRE. In either event, the magnetic field or fields generated by theconductor 300 are communicated to the MRE 250 via the continuous fluxguide (discussed below) and broken flux guide of the read head. Thetest/biasing conductor 300 includes, for example, successive layers ofMo, Au, and Mo, having thicknesses of, respectively, 0.03 μm, 0.23 μm,and 0.09 μm.

Electrical insulation for the test/biasing conductor 300 is provided bydepositing a layer 310 of electrically insulating and substantiallymagnetically impermeable material, e.g., a layer of Al₂ O₃, onto thetest/biasing conductor. The thickness of the layer 310 is about 0.3-0.9μms, preferably about 0.4 μms.

The read head further includes a continuous flux guide 320, whichoverlies the electrically insulating layer 310. As noted, the flux guide320 is shared by, and serves as the bottom pole of, the write head. Thecontinuous flux guide 320 is of magnetically permeable material, such aspermalloy, having a composition of, for example, 19.2 percent Fe and80.8 percent Ni. The continuous flux guide 320 is readily depositedusing conventional techniques, e.g., plating. The thickness of thecontinuous flux guide is about 2.0-4.0 μms, preferably about 3.0 μms.Although not show FIG. 2, the continuous flux guide 320 extends eitherinto direct physical contact with, or into close proximity to, fluxguide section 280b so as to provide a low reluctance path between thetwo flux guides.

In addition to the bottom pole (continuous flux guide) 320, the writehead of the combined read/write head 210 includes an overlying layer 330of substantially magnetically impermeable material, e.g., a layer of Al₂O₃, which defines the write gap of the present invention. The layer 330has a thickness of about 0.4-1.0 μm, preferably about 0.5-0.7 μm, and isreadily deposited using conventional techniques.

As shown in FIG. 2, the upper surface of the write gap layer 330 isnonplanar, which is a consequence of the nonplanarity introduced by thebroken flux guide of the read head. A planarization layer 340 ofsubstantially magnetically impermeable, electrically insulating materialis deposited onto the write gap layer 330. One such useful planarizationlayer is of photoresist material, such as the photoresist material soldunder the tradename AZ4340 by AZ HOECHST of Sommerville, N.J. Thephotoresist layer is readily deposited using conventionalspin-deposition techniques and has a thickness of about 2-4 μms.

An electrical conductor 350, which serves as a one-turn write coil, isreadily formed on the planarization layer 340 by depositing a layer ofelectrically conductive material, e.g., a layer of Cu or Au, onto theplanarization layer 340. The conductor 350 is readily deposited usingconventional electroplating techniques and has a thickness of about 2-4μms.

A layer 360 of electrically insulating, substantially magneticallyimpermeable material is deposited onto the one-turn write coil. Thelayer 360 is, for example, of photoresist material, as discussed above.The thickness of the layer 360 is about 2-4 μms.

The write head further includes a write top pole 370 directly overlyingthe layer 360. The top pole 370 is of magnetically permeable material,such as permalloy, the composition being, for example, 19.2 percent Feand 80.8 percent Ni. The thickness of the top pole 370 is about 2.0-4.0μms, preferably about 3.0 μms. Although not shown in FIG. 2, the rightside (as viewed in FIG. 2) of the top pole 370 extends either intodirect physical contact with, or into close proximity to, the bottompole 320 so as to provide a low reluctance path between the poles.

Other embodiments and variations of thin film heads including acomposite structure having a substantially single domain element such asan MRE on a composite insulating layer of the invention will becomeapparent to those skilled in the art and as such are intended to beencompassed within the scope of the appended claims. For example,instead to the combined read/write head described herein, the compositestructure may be incorporated into a read head.

We claim:
 1. A method of producing a composite structure, the structurecomprising a thin film structure on a substrate, the methodcomprising:forming an insulating layer on the substrate; and forming amagnetic element on the insulating layer; characterized in that: theinsulating layer is a composite layer formed by first depositing arelatively thick layer of an insulating material having surfaceirregularities, and then depositing a relatively thin layer of the samematerial on the relatively thick layer, whereby the relatively thinlayer conforms to and smooths out the surface irregularities in therelatively thick layer.
 2. The method of claim 1 in which the relativelythick and thin layers are deposited to achieve a ratio of thicknesses inthe range of about 8 to
 40. 3. The method of claim 1 in which thesurface of the relatively thick layer has indentations.
 4. The method ofclaim 3 in which the relatively thin layer is deposited to a thicknessof about 2-10 times the average depth of the surface indentations. 5.The method of claim 4 in which the relatively thin layer is deposited toa thickness of from about 0.75 to 1.5 microns.
 6. The method of claim 1in which the relatively thick and thin layers are both deposited by thesame technique.
 7. The method of claim 6 in which the relatively thickand thin layers are both sputter deposited.
 8. The method of claim 1 inwhich the relatively thick layer is polished prior to formation of therelatively thin layer.
 9. The method of claim 8 in which polishing isaccomplished by mechanical lapping.
 10. The method of claim 1 in whichthe insulating material is Al₂ O₃.
 11. The method of claim 1 in whichthe substrate is non-magnetic.
 12. The method of claim 11 in which thenon-magnetic substrate comprises TiC as a major component, and Al₂ O₃,SiO₂, and MgO as minor components.
 13. The method of claim 12 in whichthe composition of the substrate in weight percent comprises from 63-70Al₂ O₃ and 27-37 of TiC.
 14. The method of claim 1 in which thesubstrate is polished prior to formation of the relatively thick layer.15. The method of claim 1 comprising the additional steps of:(a) forminga magnetic element on the composite layer; (b) forming a secondinsulating layer on the magnetic element; (c) forming a lower flux guideon the second insulating layer, the lower flux guide consisting of twosections, a forward section and a rear section, the sections laterallyspaced apart, and extending forward and rearward from locationsoverlying end portions of the magnetic element, the lower flux guidesections and the magnetic element defining a first flux path; (d)forming a third insulating layer on the lower flux guide; (e) forming anupper flux guide on the third insulating layer, the upper flux guidedefining a second flux path; thereby forming a thin film magnetic readhead.